NGWS SDK Documentation  

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Compiler Error C3136

'interface' : a COM interface can only inherit from another COM interface, 'interface' is not a COM interface

An interface to which you applied an interface attribute inherits from an interface that is not a COM interface. A COM interface, ultimately, inherits from IUnknown. Any interface preceded by an interface attribute is a COM interface.

The following code shows how this error is generated and how you correct it.

// compile with cl /c

#include "unknwn.h"

_interface A {      // could use _interface A : IUnknown {
int a();
};

[com]
__interface B : A {
int aa();
};